A control rod is a core component with a function of controlling core reactivity by changing the number of neutrons in a core while inserting a control rod into the core and retracting a control rod out of the core. In other words, as a component used with a purpose of controlling and halting the power of a nuclear reactor, the control rod is moved up and down by a control rod drive mechanism installed on the top portion of the nuclear reactor. After receiving a fall signal from the control rod drive mechanism, the control rod freely falls down to the nuclear reactor core with the gravity. The insertion time of a control rod absolutely needs evaluation for the safe operation of a nuclear reactor and satisfaction of the allowance requirements.
A conventional control rod insertion time is calculated using one-dimensional codes. The one-dimensional codes consider factors such as hydraulic resistance of a fluid, friction, and weight of a control rod assembly, but have a drawback of a big calculation error due to no consideration of three-dimensional hydraulic effects occurring inside a guide tube for guiding the control rod. Therefore, the conventional control rod insertion time has been conservatively evaluated by applying the high uncertainly of a value bigger than a real design. Under these circumstances the control rod insertion time may be much larger than measured time during the actual operation of the nuclear reactor.
Since the control rod insertion time is used to perform the operation of the nuclear reactor and the safety analysis evaluation, a practical value needs to be deduced with elimination of exaggerated conservatism. To evaluate the control rod insertion time similar to the actually measured value in the nuclear reactor, the thermal-hydraulic phenomenon having a three-dimensional, spatial distribution of factors such as the control rod, the nuclear reactor, the guide tube, and an impact absorption tube needs to be taken into consideration.